Read-write heads for disk drives are formed at the wafer level using a variety of deposition and photolithographic techniques. Multiple sliders, up to as many as 40,000, may be formed on one wafer. The wafer is then sliced into slider bars, each having up to 60-70 sliders. The slider bars are lapped to polish the surface that will eventually become the air bearing surface. A carbon overcoat is then applied to the slider bars. Finally, individual sliders are sliced from the bar and mounted on gimbal assemblies for use in disk drives.
Slider bars are currently lapped using a tin plate charged with small diamonds having an average diameter of about 250 nm. FIG. 1A illustrates a conventional tin substrate 20 charged with diamonds 22. Top surface 24 of the tin plate typically has a certain amount of waviness. The height 26 of the diamonds 22 tends to follow the contour of the top surface 24, even after the substrate 20 is dressed. The waviness of the top surface 24 also creates a non-uniform hydrostatic film 28 during lapping operations, creating instability at the interface with the slider bars.
Conventional tin substrate is prepared in several steps. The first step is to machine a flat tin plate. The second step is to machine grooves or geometrical features that promote lubricant circulation and control the thickness of the hydrodynamic film between the oil lubricant and the slider bars.
The third step is to charge the tin plate with diamonds, such as illustrated in U.S. Pat. No. 6,953,385 (Singh, Jr.). Singh teaches applying a ceramic impregnator downward on the substrate surface with a controlled force while the diamond slurry is supplied. The diamonds are impregnated into the relatively soft tin layer of the substrate.
Fourth, the impregnated substrate is dressed with a dressing bar. The dressing bar reduces the height variation by pressing the larger diamonds further into the tin, producing a more uniform height of the diamonds. Several runs of the dressing bar help improve height uniformity of the abrasive diamonds impregnated into the tin.
FIG. 1B illustrates a conventional dressing bar 30. The leading edge 32 of the dressing bar 30 is designed with a sharp ninety-degree angle interfacing with the diamonds during the abrasive particles embedding process. The sharp leading edge 32 does not allow for efficient penetration of diamonds into the interface defined by the dressing bar and the substrate. This process generates a large amount of industrial waste. Current processes are wasteful since over 90 percent of the diamonds are lost and unrecoverable in the process.
During use, the substrate is flooded with a lubricant (oil or water based). The viscosity of oil-based lubricants is about 4 orders of magnitude greater than the viscosity of air. The lubricant causes a hydrodynamic film to be generated between the slider bar and the substrate. The hydrodynamic film is critical in establishing a stable interface during the lapping process and to reduce vibrations and chatter. To overcome the hydrodynamic film, a relatively large force is exerted onto the slider bar to cause interference with the diamonds necessary to promote polishing. A preload of about 10 kilograms is not uncommon to engage a single slider bar with the lapping media.
FIG. 2 is a schematic side sectional view of a conventional slider bar including a plurality of individual sliders before lapping. Each slider in the slider bar typically includes read-write transducers. As used herein, “read-write transducer” refers to one or more of the return pole, the write pole, the read sensor, magnetic shields, and any other components that are spacing sensitive. Various methods and systems for finish lapping read-write transducers are disclosed in U.S. Pat. No. 5,386,666 (Cole); U.S. Pat. No. 5,632,669 (Azarian et al.); U.S. Pat. No. 5,885,131 (Azarian et al.); U.S. Pat. No. 6,568,992 (Angelo et al.); and U.S. Pat. No. 6,857,937 Bajorek), which are hereby incorporated by reference.
Variables such as lapping media speed, preload on the slider bar load, nominal diamond size, and lubricant type must be balanced to yield a desirable material removal rate and finish. A balance is also required between the hydrodynamic film and the height of the embedded diamonds to achieve an interference level between the slider bar and the diamonds.
The preload applied to the slider bar is typically determined by the density of the diamonds and the diamond height variation. As the industry moves to nano-diamonds smaller than 250 nm, the preload will need to be increased to reduce the fluid film thickness a sufficient amount so the diamonds contact the slider bars. Nano-diamonds are difficult to embed in the tin plate. The risk of free diamonds damaging the slider bar increases.
Slider bars with trailing edges composed of metallic layers and ceramic layers present very severe challenges during lapping. Composite structures of hard and soft layers present differential lapping rates when lapped using conventional abrasive substrates. The variable polishing rates of the metallic and ceramic materials lead to severe recessions, sensor damage, and other problems. FIG. 3 illustrates the bar of FIG. 2 after lapping with a conventional diamond-charged substrate. The diamond-charged plates cause large transducer protrusion and recession variations, contact detection area variation, substrate recession, microscopic substrate fractures leading to particle release during operation of the disk drive, scratches from free diamonds, and transducer damage.
The realization of a data density of 1 Terabyte/inch (1 Tbit/in2) or higher depends, in part, on designing a head-disk interface (HDI) with the smallest possible head-media spacing (“HMS”). Head-media spacing refers to the distance between a read or write sensor and a surface of a magnetic media. A discussion of head-media spacing is found in U.S. patent application Ser. No. 12/424,441, entitled Method and Apparatus for Reducing Head Media Spacing in a Disk Drive, filed Apr. 15, 2009, which is hereby incorporated by reference. Conventional diamond charged plates used to lap slider bars are an impediment to achieving data densities on the order of 1 Tbit/in2.
U.S. Pat. Nos. 7,198,533 and 6,123,612 disclose an abrasive article including a plurality of abrasive particles securely affixed to a substrate with a corrosion resistant matrix material. The matrix material includes a sintered corrosion resistant powder and a brazing alloy. The brazing alloy includes an element which reacts with and forms a chemical bond with the abrasive particles, thereby securely holding the abrasive particles in place. A method of forming the abrasive article includes arranging the abrasive particles in the matrix material, and applying sufficient heat and pressure to the mixture of abrasive particles and matrix material to cause the corrosion resistant powder to sinter, the brazing alloy flows around, react with, and forms chemical bonds with the abrasive particles, and allows the brazing alloy to flow through the interstices of the sintered corrosion resistant powder and forms an inter-metallic compound therewith.
U.S. Pat. Publication No. 2009/0038234 (Yin) discloses a method for making a conditioning pad using a plastic substrate having a plurality of recesses. The abrasive grains are secured in the recesses by adhesive. The second substrate is formed around the exposed portions of the abrasive grains. After the second substrate hardens, the first substrate is removed, exposing the cutting surfaces of the abrasive grains.
Example 1 of Yin teaches recesses are about 225 micrometers deep and about 450 micrometers wide, with a maximum height difference between the highest and lowest peak of about 25 micrometers. Example 3 of Yin discloses a maximum height difference between the highest and lowest peak of about 15 micrometers. Yin discloses diamond abrasive grains with particle diameters ranging from 10 mesh to 140 mesh. Applicants believe these mesh sizes correspond generally to abrasive particles with a major diameter of about 2 millimeters to about 0.1 millimeters. The large size of the diamonds of Yin allows for insertion into the recesses. Forming the first substrate with sub-micron sized recesses and then inserting sub-micron sized abrasive grains, however, is not currently commercially viable. Sorting sub-micron sized abrasive grains is also problematic.
Other methods for orienting and positioning discrete abrasive particles are disclosed in U.S. Pat. No. 6,669,745 (Prichard et al.) and U.S. Pat. No. 6,769,975 (Sagawa), and U.S. Pat. Publication No. 2008/0053000 (Palmgren), which are hereby incorporated by reference.